pattern-memento

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• SKILL.md

SKILL.md (first lines)

# Skill: Implement Memento Pattern

## Purpose

Captures and externalizes an object's internal state without violating encapsulation, so the object can be restored to this state later.

## Procedural Instructions

1. **Define Originator:** The class whose state needs saving. It creates mementos and uses them to restore itself.
2. **Define Memento:** A value object that stores the Originator's state. It should be immutable.
3. **Define Caretaker:** Responsible for keeping track of the mementos (history) but never modifies them.
4. **Encapsulation Strategy:** Use nested classes (if supported) or restricted interfaces to keep the state private from the Caretaker.

## TypeScript Example

```typescript
class Memento {
  // Immutable
  constructor(private readonly state: string) {}
  getState() {
    return this.state;
  }
}

class Originator {
  private state: string = '';
  save(): Memento {
    return new Memento(this.state);
  }
  restore(m: Memento) {
    this.state = m.getState();
  }
}

class Caretaker {
  private history: Memento[] = [];
  // Manages the history array
}
```

## Payoff

- **Safety:** Allows producing snapshots of an object's state without violating its encapsulation.
- **Clean Originator:** Offloads history management to the Caretaker.

From johnnystefan/test-saas-business, Apache-2.0.

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